Previous studies indicated that the activation of phospholipase C and protein kinase C transduce necessary signals for exocytosis in antigen-- stimulated RBL-2H3 cells. Recent work now indicates that activation of phospholipase D may provide an additional source of messenger molecules in these cells. Phospholipase D appeared to be activated via a rise in [Ca2+]i, protein kinase C and synergistically by both mechanisms. The activation of phospholipase D, in cells stimulated with either antigen or Ca2+ ionophore resulted in sustained increases in phosphatidic acid and diglycerides. Blockade of conversion of phosphatidic acid to diglycerides with propranolol, suppressed exocytosis in A23187- and antigen-stimulated cells. Unlike phospholipase C, phospholipase D was not down regulated by the activation of protein kinase C with phorbol myristate in antigen-- stimulated cells. As these cells still exhibit exocytosis, the activation of phospholipase D may provide necessary signals for secretion.